regression:
this appears to be an attempt to compensate for fact that python's string representation of some float values having different formats:
that is in python2.5 the repr() of 1.0e-8 is actually 1e-08 (it supresses the decimal point). Since YAML does not recognize floats without the decimal point the dump command adds the float tag. However while the repr() of 1.0E-9 is 1.0000000000000001e-09. which has the decimal point making it legal YAML and it dump avoids the float tag

It seems to me that a more consistent and desirable result would be obtained if dump were to output 1e-8 as 1.0e-8 rather than using the float if one is parsing a yaml file in another language the tagged format of the float can't be read easily. (e.g. consider reading this in fortran or perl without using a yaml lib to read the file.)

A scalar value token consisting of just a period ('.') is interpreted as a float value. When construct_yaml_float() creates a float using this value, it throws an uncaught ValueError instead of yaml.YAMLError